The Swiss Federal Statistics Office reports that German is spoken by 64% of the population, French 20%, Italian 6% and 1% Romansh.
Two forms of German are used in Switzerland, High German, the same language used throughout German-speaking Europe, and Swiss-German, comprised of dozens of regional dialects unique to Switzerland, which is generally not understood by speakers of High German. High German is used mainly as the written language, found in newspapers, books, media, etc. Swiss-German, however, is rarely written and is used for everyday speaking situations. Swiss-German can be a bit difficult to pronounce, evidenced by the Swiss term for their own language, Mundart, or “mouth skill”.
Everybody writes in High German, which is also the language of all signs, official documents and public notices, but when reading out loud, people mentally transcribe the High German text into their own dialect of Swiss-German as they’re going along. Much has been written about the role of Swiss-German as an emblem and symbol of being Swiss, and how the accent of each region reflects that region’s character: the taut, stretched vowels of Baseldytsch; the slow, loping tone of Berntütsch; the clipped efficiency of Züridütsch; and so on. The Swiss are proud of their unique heritage and have no desire to erase these differences in favour of a unified language.
Swiss-French has far fewer idiosyncracies than Swiss-German. Swiss-French dialects, though still used in the hinterlands of the Jura, have virtually died out. Differences do remain from standard French, principally in accent and inflection, but you can generally speak whatever form of French you know and be both understood and respected. Indeed, in sharp contrast to France, in Romandie you can even speak English with impunity. The odd thing is that very few French Swiss speak or understand German. High German is taught in a few schools beyond elementary level, but generally only as an optional subject. English has become a more popular choice of second language. On the other hand, schools in German-speaking Switzerland almost always teach French as a compulsory subject until leaving age. As it is not generally a written language, French Swiss have virtually no opportunity to learn anything of spoken Swiss-German without going to live and work on the other side of the language border and picking it up bit by bit.
The most noticeable differences between Swiss-French and standard French are in just a very few words: instead of soixante-dix, quatre-vingt and quatre-vingt-dix, “seventy”, “eighty” and “ninety” are septante, huitante and nonante respectively. A Post Office box is called a boîte postale in France but a case postale, or CP, in Switzerland.
Similar to the situation in German-speaking Switzerland, in Italian-speaking Switzerland, written or High Italian is used less than the Lombardic dialect common to most of northern Italy. There are also about seven local Ticinese dialects, different again from each other and from Lombardic. Almost all Ticinesi are effectively quadrilingual: the local dialect is spoken to friends and family; on the street, the language of friendly conversation is Lombardic; to strangers and where there’s any element of reserve or formality, the language is High Italian; and, in addition, most Ticinesi are also proficient in German and/or Swiss-German in order to be able to communicate with the vast numbers of tourists from the north. English, although spoken by some, remains further down the list.
What this all boils down to is that, even if you happened to be fluent in Lombardic dialect, everyone you met in Ticino would anyway instinctively speak to you, being a stranger and a foreigner, in standard Italian, which happily is not excessively difficult to master.
There are five regions, each of which speak different dialects of Romansh, and they are separated from each other by regions with German-speaking populations that arrived in Switzerland in the late middle ages.
About half of the people that speak Romansh as their mother tongue no longer live in these regions, having emigrated towards the industrialised cities of German speaking northern Switzerland. In a sense, Zurich has become the biggest Romansh speaking town, however Romansh is in the same situation there as any of the other immigrant languages. There is rarely a public group consisting of people that can all understand and speak Romansh, and so they must communicate in the official regional German language.
Though Romansh has been an official language in canton Grisons for centuries, it was not officially recognized as one of the national languages of Switzerland until 1938. While the majority of official federal documents are available in German, French and Italian, only a small portion are being translated into Romansh. Whenever Romansh-speaking people leave their region, or even make a phone call outside the area, they have to be able to communicate in another language. Thus, they are all at least bilingual and are generally multi-lingual.